Horse controllers, also known as reinsmen or reinswomen, are individuals who are responsible for controlling horses. They use a variety of techniques, such as REINS, to direct the horse's movements and ensure that it is responding correctly to commands. The term 'REINS' is derived from the Old French word 'renc', which means 'to guide'. Reinsmen and reinswomen must have a deep understanding of horse behavior and be able to communicate effectively with the animal in order to ensure its safety and well-being.